Recent applications of silicon integrated circuit processing technology have led to the development and fabrication of extremely miniaturized devices. Such devices include microelectromechanical systems (MEMS) devices, which consist of an integrated microscopic-scale construction combining electrical and mechanical components. The components of such devices are typically formed and assembled on a silicon substrate using integrated circuit fabrication processes. MEMS devices can be used as switches, sensors, actuators, controllers, phase shifters, switchable/tunable filters and other integrated devices.
Due to their extremely small size, components forming part of the MEMS device can be adversely affected by external factors including RF fields, electromagnetic interference, ambient radiation, dust, gas, shock, sound waves, micro-particles, reactive gases, processing residues, moisture, and the like. To enhance performance and operating life of the device, the component can be packaged in an enclosure or encapsulation. The enclosure defines a cavity in which the component of the MEMS device can safely occupy, particularly mechanical components that need to move, such as a microresonator, for example. The enclosure functions to at least isolate the enclosed component from the external factors, to maintain the electrical connection and mounting of the components, and to permit the moving parts of the mechanical components to move freely therein. The enclosure further provides a physical barrier against shocks and rigors normally associated with handling.
The packaging of MEMS devices are typically not hermetically sealed due to high costs, and are seldom used especially among low cost commercially available packaging. In certain applications, the components of MEMS devices are maintained under specific atmospheric conditions (i.e., pressure, vacuum, temperature, gas compositions). A hermetically sealed cavity is required to sustain such conditions. Such hermetically sealed packaging is typically bulky and expensive to fabricate. Common structural bonding techniques are generally inadequate to provide good pressure sealing due to surface variations and imperfections. It is especially difficult to form a high integrity pressure seal if electrical signals must enter or exit the cavity, such as through electrical wires or feedthroughs.
Currently, components of MEMS devices requiring hermetically sealed environments are typically mounted into expensive and relatively large packages formed from multiple components of metal, ceramic or glass material that are welded or soldered together to form a sealed cavity. In one example, a preformed glass or silicate wafer cap is bonded directly onto a substrate carrying the MEMS component. During the packaging process, the glass or silicate wafer cap must be aligned carefully with the substrate to ensure proper bonding. To accommodate variations on the surface of the substrate, the package is thicker than the substrate, thus necessitating costly thinning to reduce the thickness. In addition to requiring precise alignment and thinning, the process typically exposes the MEMS device to high temperature and high voltage conditions that can undesirably damage the MEMS components. A large amount of contaminants is also undesirably generated from the bonding material used in the packaging process, which can also damage the MEMS device.
Another approach is to cap the MEMS devices either individually or in an array and form a seal with an overcoat of material. This batch packaging can lower material cost and eliminates the need for thinning. However, the time required to precisely align the caps can significantly increase the packaging costs. Using this approach, it would take over two hours to precisely align and place the caps for an array of 1-mm dies supported on a 6-inch wafer. Furthermore, the seals formed in the above processes are not generally structurally robust and thus susceptible to leakage and breakage.
Unfortunately, MEMS packaging employing evacuated cavity or pressurized cavities have not been widely adopted in industry because of the high manufacturing costs typically associated with producing MEMS with well-sealed cavities. The packaging costs of MEMS devices can range from 10 to 100 times the fabrication costs. These high packaging costs make it difficult to develop commercially viable packaged MEMS devices. Attempts to implement low cost wafer-level batch processing have typically met with failure due to device design limitations imposed by the lack of an adequate hermetic seal capable of accommodating electrical feedthroughs and wafer level batch processing methods. As a result, MEMS devices equipped with adequate pressure seal cavities are time-consuming and expensive to produce and have not been widely implemented in industry.
Therefore, there is a need for developing a process for packaging a microscopic structure to yield a microelectromechanical system (MEMS) device with an interior cavity in a cost efficient and timely manner. There is a further need to produce MEMS devices containing an evacuated cavity or pressurized cavity without degrading the packaged microscopic structure, or the overall structural integrity and performance of the MEMS device.